Fueled by electric industry deregulation, environmental concerns, unease over energy security, and a host of other factors, interest in combined heat and power (CHP), or sometimes termed as cogeneration technologies, has been growing among energy customers, regulators, legislators, and developers. CHP is a specific form of distributed generation (DG), which refers to the strategic placement of electric power generating units at or near customer facilities to supply on-site energy needs. CHP enhances the advantages of DG by the simultaneous production of useful thermal and power output, thereby increasing the overall efficiency. CHP offers energy and environmental benefits over electric-only and thermal-only systems in both central and distributed power generation applications. CHP systems have the potential for a wide range of applications and the higher efficiencies result in lower emissions than separate heat and power generation systems.
Proton exchange membrane fuel cells (PEMFCs) are highly efficient power generators, achieving up to 50-60% conversion efficiency, even at very small sizes. When combined to generate both heat and electricity, PEMFCs can reach over 80% efficiency, suggesting that CHP typically requires only ¾ of the primary energy separate heat and power systems require. This reduced primary fuel consumption is key to the environmental benefits of CHP, since burning the same fuel more efficiently means fewer emissions for the same level of output.
For typical North America residential buildings the hot water demand is seen to be smaller than the electricity demand and also fairly well correlated (temporally) with it. Waste heat from the stack appears to be well matched with the hot water demand, both in terms of magnitude and temperature. In this case, the cogeneration system typically includes a PEM fuel cell system for generating electric power and heat by reacting fuel gas with an oxidant gas, a cooling water circulating path for circulating cooling water in order to recover heat generated by the fuel cell system, a storage tank for reserving hot water to be supplied to an external hot-water supply load, and a heat exchanger for transferring heat recovered by the cooling water to water in the storage tank. Such a typical cogeneration system has been disclosed in U.S. Pat. No. 6,420,060 to Yamamoto et al. (Matsushita Electric Industrials Co.), and in JP 20022280031 to Shin et al. (Osaka Gas Co. Ltd.). Cogeneration systems of this kind usually have heat to electricity ratios of about 0.8-1.0.
JP 2003217603 to Osamu et al. (Toyota Motor Corp.) and U.S. Pat. Application No. 2002146605 to Osamu et al (AISIN SEIKI) disclose fuel cell cogeneration systems for supplying power and hot water in which an off gas combustor burns the off gas from a fuel cell anode, a first heat exchanger installed downstream a stack cooling part recovers combustion heat of the off gas as warm water, and a passage feeds the off gas residual after combustion in the off gas combustor to a reformer combustion burner. This type of cogeneration system is also often seen when an autothermal reformer is integrated with a PEM fuel cell system, such as disclosed in U.S. Pat. Application Nos. 2003064264, 2003008184 and 200218246, which usually provide thermal energy more than twice the electricity (heat to electricity ratio greater than 2). Cogeneration systems of this kind can be suitable for buildings in some European countries, in which the demand for thermal energy exceeds that for electricity.
The thermal energy recovered from a fuel cell system is sometimes insufficient in terms of hot water quantity or temperature. It is common that the space heating and electric loads are anti-correlated, with space heating demand being largest in winter when the average electric load is smallest, and vice versa. In warm weather, space heating is often not needed, and the heat generated in electricity production can be more than sufficient to heat domestic water. In cold weather, on the other hand, the heat demands of space heating and hot water can exceed the supply of energy available from the fuel cell stack plus the fuel processor assembly. There are also applications in which thermal energy demand varies largely in terms of the heat to electricity ratio, ranging between 0.5 to 2.0 or greater. For all these situations, the two typical cogeneration systems discussed above are not suitable. U.S. Pat. No. 6,290,142 to Togawa et al. (Honda Motors) discloses a cogeneration apparatus arranged to properly respond to a plurality of separate demands for supplying the thermal energy. A hot water storage tank is provided for storing a first hot water supply produced using waste heat from a fuel cell. Depending on the hot water demand, it is directly drawn from the hot water tank, while a re-heating boiler is provided for heating a second hot water supply to be provided to the thermal load of space heating.
To prevent a shortage of hot water and/or insufficient temperature and to improve the system efficiency, JP 2002289212 to Teruya and Shigetoshi (Toshiba Corp.) discloses a fuel cell cogeneration system that uses a first heat exchange device for recovering waste heat of a fuel cell and a second heat exchange device for accumulating and receiving heat from the sun, and is structured so as to store the water heated by the first and second heat exchange devices in a heat storage unit. More heat can be obtained and more water can be heated with respect to a system in which only the first device is used. Since the second device uses natural energy, supplemental heating by electric power or gas can be reduced or eliminated, so that system efficiency can be improved.
When anode off gas is recycled to the reformer burner, there are situations in which the reformer can get overheated if the fuel utilization of the fuel cell stack is lower than a specific value (e.g. <50-65%), though such low fuel utilization is often unavoidable in situations such as start up and load changes. To prevent the reformer from overheating, JP2002042840 to Hiroki and JP 2003217603 to Osamu et al. discloses a fuel cell cogeneration system with a mechanism feeding the anode off gas of the fuel cell to both a reformer combustor and an auxiliary cogeneration water-heating burner. A gas flow meter detecting the anode off gas flow rate is provided in an anode gas outlet of the fuel cell.
U.S. Pat. No. 6,861,169 to Michael et al. discloses a cogeneration system in which one of the embodiments proposes the use of an optional furnace, to provide heat for at least one of space heating and potable water heating. There may also be a controller for providing sufficient heat for at least one of space heating and potable water heating by selectively activating at least one of the additional furnaces and the burner in the fuel cell and fuel processor assembly. The additional furnace may also optionally be used to provide supplemental heat to the third fluid loop for use in one or more of space heating, potable water heating, and provision of startup heating to the cogeneration system.
U.S. Pat. No. 5,335,628 to Dunbar discloses a fuel cell and a boiler coupled in such a manner that the water used to capture excess heat generated by the fuel cell is used for boiler feed water heating. In one embodiment, steam generated by the boiler is used in an operation that converts the steam to condensate, and the liquid is returned to the fuel cell for use as a heat sink for thermal energy generated within the fuel cell unit.
Obviously, there is a need to provide an efficient and effective combined heat and power (CHP) or cogeneration system. In particular, it is needed to provide a CHP system that can be operated in a simple manner to produce both thermal and electric energies, and have the heat to electricity ratio in a wide range, say from 0 to more than 2, to be applicable for a wide practical applications characterized by different demands of hot water, space heating and electric power.